Move cursor to first line of the group you want to write. Hit m
and a sequentiall. That's "set mark named 'a'".

Move cursor to last line of the group, hit 'm' and 'b'.'

Change over to command mode hit: as a sequence do :'a,'b w
filename then hit return.

That will work in vi, nvi and vim.

Another method, works in more modern vim:

Put cursor on first line of the group of lines you want to write out. Hit V, for "start visual block of lines".

Move cursor to the bottom of the group of lines. Vim will highlight with reverse video each line.

Write out. Hit : to go into command line mode. That will give you a prompt at the bottom of the scren that looks like: `:'<,'>" That means from the start of the visual block to the end. Type the filename and hit return.

You can add singled lines to the end of the file as you need to by putting your cursor on the line you want to write out then typing: :.w >> filename and hitting return.

The "write a group of lines" commands also work with ">> filename" instead of just "filename". The ">>" causes vi or vim to append to the file, rather than just writing it.